Urethane prepolymers have been modified in the past to improve or add to their basic properties by end-capping some or all of the isocyanate groups with various organofunctional silanes. Among these methods, U.S. Pat. No. 3,632,557 teaches the complete end-capping of conventional polyurethane prepolymers with primary and secondary aliphatic aminosilanes. U.S. Pat. No. 3,979,344 details a room temperature curable silicon terminated organic sealant composition comprising a small quantity of 3-(N-2-aminoethyl)aminopropyltrimethoxysilane endcapper to improve the sealant's cure speed. U.S. Pat. No. 4,222,925 details the same composition as is given in U.S. Pat. No. 3,979,344 but also incorporates a reinforcing carbon black filler. Sealants having improved elongation and flexibility were prepared using silane end-capped polyurethane polymers obtained from silane monomers having at least one dialkoxy group and an organofunctional group with at least one active hydrogen atom in U.S. Pat. No. 4,645,816.
However, polyurethane prepolymers end-capped with primary aminofunctional silanes contain an active hydrogen atom which is capable of further reactions with the isocyanate end groups. This reactivity can lead to undesirable stability performance of both the polymer and sealant compositions. Several methods teach the use of secondary aminofunctional silane endcappers. For example, U.S. Pat. No. 4,374,237 teaches the curable isocyanate terminated polyurethane prepolymers at least part of whose terminal isocyanate groups have been reacted with a secondary amine containing silane monomer having two trialkoxy silane groups. U.S. Pat. No. 4,474,933 describes crosslinkable polyurethane resin mixtures that have been capped with various primary and secondary difunctional aminosilane endcappers. Other references of interest relating to silane end-capped urethane prepolymers and sealants are U.S. Pat. No. 3,627,722; U.S. Pat. No. 4,067,844; U.S. Pat. No. 4,625,012; U.S. Pat. No. 4,345,053; U.S. Pat. No. 4,645,816; and U.S. Pat. No. 5,354,880).
More recently, U.S. Pat. No. 5,364,955 teaches the use of N-alkoxysilylalkyl aspartic acid esters, and discloses the reaction of the dimethyl and diethyl esters with a diisocyanate prepolymer and the further reaction of this product so as to form a polymeric film on a glass surface. This patent does not disclose, let alone quantify, any properties of the polymeric film thus formed other than that it is opaque and elastic. The present applicants have determined, however, that sealants prepared from polyurethane capped with the dimethyl ester disclosed in this patent exhibit only mediocre performance in such significant properties as thermal stability, elongation, tensile strength, and tear resistance, from which one would expect the same mediocre performance from the other esters disclosed in this patent.
Therefore, it is an object of the present invention to provide compositions useful for instance as sealants, coatings, and the like, based upon urethane prepolymers end-capped with certain N-alkoxysilylalkyl aspartic acid esters, wherein the compositions upon curing form a product having superior elongation, tensile strength and tear resistance in comparison to formulations based on other N-alkoxysilylalkyl aspartic acid ester end-capped polymers.